The invention relates to multimode multi-track optical reading and recording using multimode laser diodes.
Laser diodes are available as single mode or multimode diodes. Single mode laser diodes are effectively point sources and are diffraction limited in their divergence on all axes. In contrast, multimode diodes typically have laser junctions in the form of short stripes and are often referred to as xe2x80x9cstripexe2x80x9d type laser diodes. Multimode laser diodes are diffraction limited in the direction perpendicular to the junction (their short axis), but have non-diffraction limited divergence in the parallel to the laser junction (their elongated axis).
The emitting aperture of a multimode diode can be a single or continuous stripe, a collection of short stripes or even a collection of single mode emitters electrically connected in parallel. In this application, the phrases xe2x80x9cmultimode diodexe2x80x9d and xe2x80x9cmultimode laser diodexe2x80x9d should be understood to incorporate each of these different diode constructions. In addition, laser diodes can emit radiation of various different frequencies and any reference to xe2x80x9clightxe2x80x9d in this application should be understood to incorporate any radiation frequency.
For recording applications, the principal advantage of using multimode laser diodes is that the radiation emitted from multimode diodes can be of substantially higher power than that emitted from single mode diodes. Obviously, higher power is a desirable quality for a recording operation, where heat or optical power alter the physical characteristics of the recording media. Despite this advantageous characteristic, a difficulty associated with multimode laser diodes is their non-uniform near-field power distribution. Not only is the near-field power distribution of a multimode diode non-uniform, but it typically changes with the age and usage of the diode. In an optical recording device, non-uniformity of the near-field power distribution can result in an unacceptable non-uniformity at the recording media, where the image is to be recorded. As a result, the image may be significantly degraded. This degradation may in turn lead to data loss or corruption. Therefore, most optical recording devices use single mode laser diodes, despite their relatively low power.
Accordingly, an apparatus and method are required to improve the performance of multimode laser optical recording devices. Such an improvement would overcome non-uniformity in the near-field power distribution of the multimode laser diode and faithfully record images on a recording media without banding, data loss or data corruption.
FIG. 1 depicts a prior art optical read/write head implemented using a light valve 3. Light from the laser diode source 9 is imaged via lens 10 into a line 12 on the light valve 3. The light valve 3 separates the light of the line 12 into a plurality of individually controllable component beams or xe2x80x9cchannelsxe2x80x9d (not shown). The individually controllable channels from the light valve 3 are imaged through polarizing beam splitter 4, quarter waveplate 5, and lens 6 onto the recording media 7. Lens 6 is typically an aspheric lens of high numerical aperture. An autofocus system (not shown) may be used to overcome the shallow depth of focus of the lens 6. The area 2 on the recording media 7 represents the image of the line 12 from the light valve 3. Within the area 2, the channels from the light valve 3 are imaged to form a plurality of tracks 11. Depending on the resolving power and/or data storage needs of the optical system, each parallel track 11 on the recording media 7 may comprise one or a plurality of channels.
During a write operation, the laser diode 9 is operated at high power and the individual channels from the light valve 3 are digitally controlled, so as to reproduce an image on the recording media 7. Typically, the recording media 7 and the optical recording system are moved or xe2x80x9cscannedxe2x80x9d relative to one another. The arrows on the recording media 7 of FIG. 1 indicate the scanning direction. While scanning, the data modulated into individual channels are controlled so as to record that data in the tracks 11 on the recording media 7.
In a read operation, the laser diode 9 is operated at low power (or a separate read diode is used). The light valve 3 causes the entire line 12 to be directed toward area 2 of the recording media 7 (i.e. the light valve is configured such that each of the channels are in an xe2x80x9conxe2x80x9d state). The linearly polarized light from the laser diode 9 is converted to circular polarization by the quarter waveplate 5 before it impinges on the area 2 of the recording media 7. Depending on whether a particular track 11 within the area 2 has been recorded on or not, the intensity profile of the light reflected from the area 2 will vary. Upon reflection from the area 2, the beam is converted back to linear polarization by the waveplate 5, but the axis of polarization of the reflected beam is orthogonal to that of the incident beam. Consequently, when the reflected beam impinges on the polarizing beam splitter 4, it is reflected again onto detector array 8. Differences in the power distribution of the reflected beam at the detector array 8 allow the data in each track 11 of the recording surface 7 to be xe2x80x9creadxe2x80x9d electronically.
A multi-track optical recording system may incorporate xe2x80x9cmark lengthxe2x80x9d modulation, xe2x80x9cmark widthxe2x80x9d modulation or both. These modulation schemes are explained in U.S. Pat. No. 5,802,034. U.S. Pat. No. 5,802,034 has the same inventor and assignee as the present invention and is hereby incorporated by reference.
Typically, multi-track optical recording systems use single mode laser diodes, because of the problems caused by the non-uniformity in the near-field power distribution of multimode lasers.
Accordingly, an apparatus and method are required to provide improved multi-track optical recording devices using multi-mode laser diodes. Such an improvement would overcome non-uniformity in the near-field power distribution of multimode laser diodes and faithfully record data and images on a recording media without banding, data loss or data corruption.
The present invention discloses an optical recording system, which comprises several elements. The recording apparatus includes a multi-channel light valve, a multimode radiation source having a short axis and an elongated axis and an optical subsystem with at least one optical element. The optical subsystem is operative to direct radiation from the radiation source to the light valve. Furthermore, the optical subsystem is anamorphic, introducing astigmatism, such that radiation directed from the radiation source onto the light valve is substantially focussed on the short axis and is less focussed on the elongated axis. The invention additionally comprises a recording medium, which may be permanently marked in response to incidence of imaging radiation from the light valve. Finally, an imaging assembly is located in an optical path between the light valve and the recording medium, and is operative to focus radiation from the light valve in such a manner to record permanent image marks on the recording medium.
Preferably, the radiation, which is incident on the light valve via the optical subsystem may be substantially uniform over its elongated axis.
Advantageously, the optical subsystem may comprise at least one cylindrical lens.
Preferably, the light valve may be made of either micromachined silicon or electro-optical material.
Preferably, the radiation source may be a multimode laser diode.
Advantageously, the optical recording system may be operative to simultaneously record a plurality of data tracks on the recording medium.
Another aspect of the present invention involves a similar optical recording system, wherein the optical subsystem is operative to direct radiation from the radiation source to the light valve and to substantially focus the radiation on the short axis, while ensuring that the radiation on the elongated axis is less focussed. Advantageously, this optical subsystem may comprise a cylindrical lens, a microlens array, a microprism array, or a grating.
Another aspect of the present invention involves a similar optical recording system, wherein the optical subsystem is operative to direct radiation from the radiation source to the light valve and to substantially focus the radiation on the short axis, while blurring the radiation on the elongated axis.
Another aspect of the present invention involves a method of reducing non-uniformity in a multimode optical recording system. The optical recording system utilizes a multi-channel light valve, which receives radiation from a multimode radiation source having a short axis and an elongated axis. The method comprises the step of introducing astigmatism between the radiation source and the light valve, such that the radiation received by the light valve is substantially focussed on the short axis and is less focussed on the elongated axis.
Another aspect of the present invention involves a similar method of reducing non-uniformity in a multimode optical recording system. It comprises the step of introducing at least one optical element between the radiation source and the light valve, such that the radiation received by the light valve is substantially focussed on the short axis and is less focussed on the elongated axis.
Another aspect of the present invention involves a similar method of reducing banding in a multimode optical recording system, which comprises the steps of:
(a) focussing an image of the short axis of the radiation source onto the light valve; and
(b) causing an image of the elongated axis of the radiation source to be blurred on the light valve.